This invention relates to x-ray scintillator compositions for x-ray imaging applications.
X-ray imaging systems have been used for a variety of different applications, including therapeutic and diagnostic applications, such as computerized tomography (CT) and digital radiography (DR).
Computerized tomography involves exposing a subject to a relatively planar beam or beams of x-ray radiation. By measuring the x-ray intensity (i.e., the x-ray absorption) along a plurality of different angles or views, x-ray absorption coefficients may be computed for various areas in any plane of the body through which the radiation passes. The absorption coefficients may be used to produce an image of the object or objects (e.g., the bodily organs of a human subject) being intersected by the x-ray beam. Radiation therapy involves delivering a high, curative dose of radiation to a target (e.g., a tumor), while minimizing the dose delivered to surrounding healthy tissues and adjacent healthy organs.
Diagnostic and therapeutic radiation may be supplied by a charged particle accelerator that is configured to generate a high-energy (e.g., several MeV) electron beam. The electron beam may be applied directly to one or more target sites on a patient, or it may be used to generate a photon (e.g., X-ray) beam, which is applied to the patient. An x-ray tube also may supply therapeutic photon radiation doses to a patient by directing a beam of electrons from a cathode to an anode formed from an x-ray generating material composition. The shape of the radiation beam at the target site may be controlled by discrete collimators of various shapes and sizes or by multiple leaves (or finger projections) of a multi-leaf collimator that are positioned to block selected portions of the radiation beam. The multiple leaves may be programmed to contain the radiation beam within the boundaries of the target site and, thereby, prevent healthy tissues and organs located beyond the boundaries of the target site from being exposed to the radiation beam.
An integral part of diagnostic and therapeutic x-ray imaging systems is the x-ray detector that receives the x-ray radiation, which has been modulated by passage through the body being examined or treated. The x-ray detector generally includes a scintillator material that emits optical wavelength radiation when excited by the impinging x-ray radiation. In typical medical or industrial applications, the optical output from the scintillator material impinges upon a photodetector array that produces electrical output signals corresponding to the optical radiation received from the excited scintillator material. The amplitude of the output signals is related to the intensity of the impinging x-ray radiation. The electrical signals may be digitized and processed to generate absorption coefficients in a form suitable to display on an imaging screen or on a recording medium.
In x-ray diagnostic and therapeutic applications, it is highly desirable to reduce the scan time as much as possible because, by reducing the scan time, a larger area of the patient may be covered in a given time (e.g., a single breath hold) and the cumulative radiation dose delivered to the patient (e.g., during positioning) may be reduced. Shorter scan times also reduce image blurring that might be caused by movement of the patient and internal organs. In general, the scan time of an x-ray imaging system is determined, at least in part, by the decay time of the scintillator.
In addition to having a fast decay time, an x-ray imaging scintillator should have a number of other properties. For example, the scintillator should be an efficient converter of x-ray radiation into optical radiation in a wavelength range that is most efficiently detected by the photodetector array of the x-ray detector. It is also desirable for the scintillator to transmit optical radiation efficiently. In addition, the scintillator material should have high x-ray stopping power, low hysteresis, spectral linearity, and short afterglow. High x-ray stopping power is desirable for efficient x-ray detection, because x-rays not absorbed by the scintillator escape detection. Hysteresis refers to the scintillator material property whereby the optical output varies for identical x-ray excitation, based on the irradiation history of the scintillator. Spectral linearity is important because x-rays impinging on the scintillator body typically include a number of different energies, and because the scintillator response to the radiation should be substantially uniform for all such energies. Afterglow is the tendency of the scintillator to continue to emit optical radiation for a period of time after the x-ray excitation has terminated. Long afterglow tends to blur the information-bearing signal over time. Furthermore, for applications requiring rapid sequential scanning (e.g., applications in which moving bodily organs are imaged), short afterglow is essential for rapid cycling of the detector.
The invention features inventive scintillator compositions that are useful for x-ray imaging applications in general, and that are particularly suited for x-ray imaging applications in which fast scan times are desired.
In one aspect, the invention features an x-ray imaging system that includes a scintillator that comprises praseodymium (Pr) doped gadolinium oxysulfide (Gd2O2S) and a detector array that is positioned adjacent to the scintillator.
In a preferred embodiment, the scintillator comprises Gd2O2S doped with a Pr concentration of 0.5-2.5 mole percent, a coating weight in the range of 30 mg/cm2 to 150 mg/cm2, and a particle size in the range of 7-10 xcexcm.
The detector preferably comprises an array of amorphous silicon photodetector cells.
The invention also features a linear accelerator based x-ray imaging system and method, each of which incorporates the above-mentioned praseodymium-doped gadolinium oxysulfide scintillator.
Among the advantages of the invention are the following.
The use of the novel praseodymium (Pr) doped gadolinium oxysulfide (Gd2O2S) scintillator compositions of the invention enables image artifacts that otherwise might be caused by the pulse nature of linear accelerator based x-ray radiation sources to be reduced substantially, while maintaining relatively fast imaging scan times.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.